The invention generally relates to methods for producing large quantities of substantially monodisperse droplets for use in capsule-based electrophoretic displays. More particularly, the methods relate to producing substantially uniformly-sized droplets of a first phase, the first phase including a fluid and particles, for introduction into a second phase, or the methods relate to producing substantially uniformly-sized complex droplets having a core formed from a first phase, the first phase including a fluid and particles, and a second phase that surrounds the first phase as a shell.
Traditional emulsification methods are not ideally suited for forming capsules to be used in electrophoretic displays. Current methods produce disperse phase droplets that are smaller than the desired size range. For example, while some systems produce droplets as large as tens of micrometers in diameter, typical droplets are of the order of 0.01 xcexcm to 1 xcexcm. Furthermore, many traditional emulsification techniques result in polydisperse emulsions, i.e., emulsions that are not characterized by a narrow drop size distribution. Thus, a need exists to produce substantially uniformly-sized droplets for forming capsules to be used in electrophoretic displays.
Methods of the invention can produce large quantities of substantially uniformly-sized droplets or complex droplets for forming capsules useful for electrophoretic displays. Moreover, methods of the invention can produce a group of substantially uniformly-sized droplets from a first phase containing both a fluid and plurality of particles. These droplets are applied to a second phase. Once in contact with the second phase, any of a variety of steps can be performed, including encapsulating the droplets. Alternatively, methods of the invention can produce a group of substantially uniformly-sized complex droplets for forming capsules useful for forming electrophoretic displays. The complex droplets are formed from a first phase, containing both a fluid and a plurality of particles, at their core and a second phase that surrounds the first phase as a shell. Typically, the core of the complex droplet also is a substantially uniformly-sized droplet relative to the other cores in the group of complex droplets.
In one aspect of the invention, a method for forming substantially uniform droplets includes the steps of providing a non-aqueous internal phase; providing an external phase; vibrating the internal phase; and applying the internal phase to the external phase. The internal phase includes a plurality of particles suspended in a first fluid; the external phase includes a second fluid; and a series of droplets of substantially uniform size are formed. The droplets can be formed from the internal phase, or the droplets can be formed from both the internal and external phases.
This aspect of the invention can have any of the following features. The first fluid can be an oil. The second fluid can be an aqueous solution. The step of applying the internal phase to the external phase can include having the internal phase contained within a structure and pressurizing the internal phase so that the internal phase issues from the structure into the external phase. During the step of applying described above, the internal phase can issue through at least one aperture; can issue in at least one train of droplets; and/or can be applied to the external phase at a plurality of locations. A droplet can have a diameter of about 20 xcexcm to about 300 xcexcm and can have a substantially uniform size relative to other droplets in the series of droplets. The step of vibrating the internal phase can include vibrating the internal phase with a vibrating member. The vibrating member can be a piezoelectric transducer. Alternatively, an electro-mechanical or magnetostrictive or other similar vibrating member can be used. The step of vibrating the internal phase can include vibrating a conduit containing the internal phase and/or the internal phase can issue from the conduit in two or more trains of droplets, and/or a tip of the conduit, through which the internal phase issues into the external phase, can be in communication with the external phase. The step of applying the internal phase to the external phase can include simultaneously issuing the internal phase and the external phase through two adjacent channels. These two adjacent channels can terminate at two concentric nozzles. The method can further include the step of mixing the particles with the first fluid. Mixing can be accomplished by inducing a flow within the internal phase.
In another aspect of the invention, a method for forming substantially uniform droplets includes the steps of providing a non-aqueous internal phase; providing an external phase; and applying the internal phase to the external phase through an aperture in a container. The internal phase includes a plurality of particles suspended in a first fluid and the external phase includes a second fluid. The internal phase is moved relative to the external phase such that as the internal phase contacts the external phase a droplet separates from a remainder of the internal phase and such that a series of droplets of substantially uniform size is formed. This aspect of the invention can further include the step of vibrating the internal phase.
In another aspect of the invention, a method for forming substantially uniform droplets includes the steps of providing a non-aqueous internal phase; providing an external phase; and applying the internal phase to the external phase. The internal phase includes a plurality of particles suspended in a first fluid; the external phase includes a second fluid; the internal phase is pressurized and pulsed through a valve such that the internal phase forms a series of droplets of substantially uniform size.